1. Field of the Invention
The present invention relates to a Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet and an electrolyte for manufacturing the same, and more particularly to a Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet with excellent corrosion resistance and weldability, and an electrolyte for manufacturing the same in a stable manner.
2. Description of the Prior Art
In recent years, zinc plated steel sheet has been favored over competing steel sheets having other corrosion resistant surfaces and has been widely used for automobiles, household electric appliances and construction materials because of its excellent corrosion resistance. However, new plated steel sheets with high corrosion resistance by coatings in the form of a thin film are now required in terms of energy and resource savings. To meet these requirements, a Znxe2x80x94Fexe2x80x94and a Znxe2x80x94Nixe2x80x94 alloy electroplated steel sheet have been developed and now are commercially available. In addition, a Znxe2x80x94Cr alloy plated steel sheet has been developed.
However, as for a Znxe2x80x94Fe alloy plated steel sheet, iron is contained in a plating layer which is formed on the steel sheet. Therefore, when the steel sheet is exposed to a corrosive atmosphere, the plating layer protects the steel sheet by sacrificial anticorrosive reaction. However, the plating layer is dissolved and then iron therein is oxidized to produce a red corrosive product. Final consumers consider it as a rusted steel sheet and thus tend to avoid the use of the Znxe2x80x94Fe alloy plated steel sheet. Furthermore, there is a disadvantage in that ferrous ion is oxidized to ferric ion, thereby forming sludge during manufacturing the Znxe2x80x94Fe alloy plated steel sheet.
As for a Znxe2x80x94Ni alloy plated steel sheet, it is widely used as a material of automobiles due to its excellent corrosion resistance. However, because humans have been known to experience an allergic reaction to nickel, use of nickel-plated products has been prohibited in Europe. This trend continues to spread all over the world.
A Znxe2x80x94Cr alloy plated steel sheet is markedly better than a Znxe2x80x94Fe or a Znxe2x80x94Ni plated steel sheet in terms of corrosion resistance and can obtain desired corrosion resistance even though a thin film of plating is applied to the steel sheet. In spite of these advantages, plating efficiency is low and thus production cost is high. Furthermore, chromium is harmful in the human body and thus its use is prohibited according to the environmental acts and regulations. Accordingly, it is difficult to use it practically.
As for a chromate surface treated steel sheet to improve corrosion resistance of zinc plated steel sheets, it does not have sufficient corrosion resistance to be used as a steel sheet for automobiles. Besides, chromium on the surface of the steel sheet is vaporized during a process of manufacturing automobiles, thereby causing damage to the human body. Therefore, its use has now been prohibited.
Accordingly, there is need to develop new alloy plated steel sheets with excellent corrosion resistance which are less hazardous to the human body.
An alloy electroplated steel sheet developed for these purposes is disclosed in U.S. Pat. No. 3,791,801. The patent discloses an electroplated steel sheet with excellent corrosion resistance, in which 0.05-2 weight % of one or more of molybdenum oxides or tungsten oxides are present in a zinc plating layer, or 0.05-2 weight % of one or more of molybdenum oxides or tungsten oxides and 0.5-15 weight % of metals or oxides of Fe, Ni, Co, Sn, Pb, etc. are co-deposited in the zinc plating layer.
In the above U.S. Pat. No. 3,791,801, the molybdenum and tungsten are present as their colloidal oxides in an acidic electrolyte. The molybdenum and tungsten oxides are physically filled in or are adsorbed chemically on a plating layer during plating, thereby being present as oxides such as MoO2, Mo2O3, WO2 and W2O3, or hydroxides in the plating layer. If the molybdenum or tungsten oxide is present in the plating layer, the oxide is effective to restrain the dissolution of zinc under corrosive conditions, thereby enhancing corrosion resistance. In addition, if the oxide is present on the surface of the plating layer, the oxide adheres tightly to the paint layer, ensuring excellent adhesion after painting
A zinc plated steel sheet comprising oxides such as MoO2, Mo2O3, WO2, W2O3, etc. in a plating layer, is known to be useful in electric household appliances requiring corrosion resistance and paintability. However, because Mo or W is not present as alloy with zinc but is individually present as an oxide, when spot welding is carried out, for example, in a car body, there is a problem in that the oxide present on the surface of the plating layer can inhibit the flow of current and thus lower weldability. In particular, recently, automobile manufacturing companies have used mainly a projection welding method, in which several electrode tips are mounted, thereby being capable of carrying out the spot welding at the several electrode tips at one time. In this case, electric resistance must be uniform throughout a steel sheet, so that uniform welding at all electrode tips is accomplished.
Accordingly, as for a steel sheet, in which oxide such as MoO2, Mo2O3, WO2 or W2O3 or the like is present at the steel sheet surface and a plating layer which is formed on the steel sheet, due to high electric resistance of the plating layer, it is necessary to apply higher current for normal welding. However, when higher current is applied, zinc is quickly vaporized, so as to shorten the life of electrode tips; at the same time, spatter, a phenomenon that small droplets are dispersed to adjacent areas, is generated.
Furthermore, when there are differences in oxide content among sections of the steel sheet, electrical resistance among the sections is different. As a result, current passes toward the electrode tips with low electrical resistance during projection welding, but does not pass toward the electrode tips with high electrical resistance. Consequently, welding is not achieved.
Still furthermore, when oxide such as MoO2, Mo2O3, WO2, or W2O3, or the like is present in a zinc plating layer, binding force of metallic zinc with the oxide is weak. As a result, a plating material is stripped off as fine powder during bending of the steel sheet. This phenomenon is what is called xe2x80x9cpowderingxe2x80x9d. When powdering occurs, there is a problem in that after the bending, the sections where powdering occurs are not provided for the substrate protection, thereby lowering corrosion resistance.
Meanwhile, Japanese Patent Laid-Open Publication No. 57-114686 discloses a technique, by which citric acid, formic acid and tartaric acid are added to an acidic electrolyte, to prevent tungsten and molybdenum, etc. from forming colloidal oxide sludge in the acidic electrolyte. As described in the publication, a glossy zinc electroplated steel sheet is manufactured using an electrolyte containing zinc ion as a main component, one or more of Co, Mo, Ni, Fe, Cr, W, V, In, Sn, and Zr ions, and an organic additive. Addition of citric acid, formic acid, and tartaric acid to the electrolyte makes it possible to prevent sludge formation of the metal ions and thus to reduce the amount of the sludge. As a result, workability in plating is enhanced and loss of effective metal ions in the electrolyte is reduced.
As can be seen from the FIG. 1 and FIG. 2 in the publication, the amount of sludge was reduced in the electrolyte containing citric acid, formic acid, and tartaric acid, compared with the electrolyte in the absence of them. However, the formation of sludge was not completely prevented.
Generally, when a steel strip is continuously electroplated, as plating proceeds, the concentration of a variety of metal ions in an electrolyte is reduced. Therefore, the reduced amount must be replenished, so that a plating layer with uniform metal ion content can be stably formed on a steel sheet.
As for the above publication, in the same manner, as plating proceeds, the concentration of metal ions in an electrolyte is reduced. In order to maintain the concentration uniformly, metal salts must be injected from the outside periodically. However, whenever the metal salts are injected, a small amount of sludge is formed. The formed sludge must be removed by a filter apparatus or by a dissolution apparatus. Unless the sludge is removed, it continues to be present in an electrolyte. Consequently, as plating proceeds, an amount of the sludge in the electrolyte becomes large.
When colloidal tungsten oxide is even slightly present in an electrolyte for manufacturing a Znxe2x80x94Coxe2x80x94W alloy plated steel sheet, there are problems in that the tungsten oxide is co-deposited on a plating layer, thereby deteriorating weldability, and the binding force of the tungsten oxide with metallic zinc and/or cobalt is weak, thereby the plating layer being stripped off in the form of powder during bending. As described in the above, the technique presented in the above publication cannot completely prevent formation of sludge in an electrolyte. As a result, tungsten oxide, etc. is electrodeposited on the plating layer during electroplating, and thus a plated steel sheet with excellent corrosion resistance and weldability cannot be efficiently manufactured.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet with excellent corrosion resistance and weldability, in which by alloy plating the steel sheet with metallic zinc, cobalt and tungsten in an appropriate ratio, a plating layer consisting of zinc, cobalt and tungsten is formed on the steel sheet.
It is another object of the present invention to provide an electrolyte for manufacturing the Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet, in which the steel sheet is plated with metallic zinc, cobalt and tungsten in an appropriate ratio, whereby tungsten component of the alloy is metallic tungsten.
As for tungsten ion in an aqueous solution, it is known that WO42xe2x88x92 ion is stable in pH of about 7 or more, (HW6O21)5xe2x88x92 where pH is 4-7, (H3W6O21)3xe2x88x92 where pH is 3-4, and (W12O39)6xe2x88x92 where pH is 3 or less. When tungstate, Na2WO4, K2WO4 or (NH4)2WO4 is dissolved and stored in water, as time goes by, sludge is generated at pH 4 or less. As pH is lower, sludge generation velocity increases.
If the tungstate is dissolved in an electrolyte containing Zn ion and Co ion, unlike in water, sludge is generated above pH 3. As pH is higher, sludge generation velocity increases. We assume that the tungstate binds with Zn2+ and Co2+ ions to generate sludge. However, the complete reaction mechanism is not known. Accordingly, when electroplating is carried out in a plating bath containing Zn ion, Co ion and W ion below pH 6, tungsten oxide such as WO2 or W2O3 is applied to a plating layer, as described in the U.S. Pat. No. 3,791,801.
On the other hand, where tungsten is present as oxide on a plating layer, the tungsten oxide on the surface of the plating layer adheres closely to the plating layer, ensuring excellent adhesion. However, where the oxide is present inside the plating layer, the stress of the plating layer becomes large and the binding force of the oxide with plating layer components becomes weak. As a result, the plating layer tends to be stripped off during bending of a steel sheet and spot weldability is poor.
Accordingly, the inventors have repeatedly studied and experimented so as to plate a steel sheet with metallic tungsten, using a zinc-cobalt-tungsten plating alloy. As a result, they found that when citric acid was added to an electrolyte in an appropriate amount, almost all of the tungsten formed a complex compound with the citric acid, thereby permitting the plating to be carried out with metallic tungsten in contrast to tungsten oxide.
In accordance with one aspect of the present invention, there is provided a Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet with excellent corrosion resistance and weldability, in which a plating layer consisting of Co: 0.1-3.0 weight %, W: 0.1-2.0 weight % and Zn: balance, is formed on the steel sheet, and the tungsten component of the alloy is metallic tungsten.
In accordance with another aspect of the present invention, there is provided an electrolyte for manufacturing the Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet, comprising zinc chloride: 60-200 g/l, cobalt chloride: 0.1-6.0 g/l, tungsten: 0.1-4.0 g/l, citric acid: 0.5-10.0 g/l, polyethylene glycol: 0.1-2.0 ml/l and electric conductive aid: 30-400 g/l, in which almost all ions of the tungsten form a complex compound with the citric acid, thereby preventing formation of sludge.
In accordance with yet another aspect of the present invention, there is provided a Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet, on which a plating layer is formed by Znxe2x80x94Coxe2x80x94W alloy electroplating using the electrolyte.
Hereinafter, the electroplated steel sheet and electrolyte of the present invention will be illustrated by way of detailed description and examples which are non-limiting to the spirit and scope of the invention.
Electroplated Steel Sheet
As described in the above, as for the Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet of the present invention, the plating layer consists of Co: 0.1-3.0 weight %, W: 0.1-2.0 weight %, and Zn: balance, and the tungsten component of the alloy is metallic tungsten.
That is, the content of cobalt present in the plating layer of the present invention is defined as 0.1-3.0 weight % (hereinafter, % only) as calculated for metallic cobalt. If the content of cobalt is 0.1% or less, corrosion resistance is poor, while if it exceeds 3.0%, corrosion resistance is excellent but the cost of cobalt is high. Therefore, the use of more than 3.0% cobalt is not economical.
The content of the tungsten present in the plating layer of the present invention is defined as 0.1-2.0%. If the tungsten content is 0.1% or less, corrosion resistance is poor, while if it exceeds 2.0%, powdering occurs on the plating layer.
The present invention is characterized in that the tungsten is present in the plating layer in the form of metallic tungsten, not tungsten oxide. The tungsten, along with the zinc and cobalt, forms an alloy. As a result, corrosion resistance is enhanced and spot welding is improved.
As described above, if 0.1-3% of metallic cobalt and 0.1-2.0% of metallic tungsten are present in the plating layer, a Znxe2x80x94Coxe2x80x94W alloy present in the plating layer acts as a barrier against corrosion, ensuring markedly improved corrosion resistance. At the same time, all the tungsten in the plating layer is present as metallic tungsten, and thus spot weldability is excellent and powdering, heretofore caused during forming operations, does not occur.
Electrolyte
Hereinafter, an electrolyte for manufacturing the Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet of the present invention will be described in detail.
The concentration of zinc chloride in the electrolyte of the present invention is defined as 60-200 g/l. If the concentration of zinc chloride is 60 g/l or less, a continuous high current density plating is impossible, while if it exceeds 200 g/l, zinc chloride remains undissolved, thereby zinc salt being deposited.
The concentration of cobalt chloride in the electrolyte is defined as 0.1-6.0 g/l. When the cobalt chloride is at least 0.1 g/l, the cobalt content in the electrolyte can be stably maintained above 0.1%. The reason why the upper limit is 6.0 g/l, is that the value is sufficient for obtaining 0.1-3 wt % of cobalt content in the plating layer.
The electrolyte of the present invention comprises 0.1-4.0 g/l of tungsten. When the tungsten concentration is at least 0.1 g/l, the tungsten content of 0.1% or more is stably secured in the plating layer. Furthermore, the reason why the upper limit is 4.0 g/l, is that the value is sufficient for obtaining 0.1-2% of tungsten content in the plating layer.
In accordance with the present invention, the tungsten is preferably added in the form of one or more soluble tungstates selected from sodium tungstate, ammonium tungstate and potassium tungstate.
The electrolyte of the present invention comprises 0.5-10.0 g/l of citric acid. Preferably, the citric acid is added in the form of one or more soluble citrates selected from sodium citrate, ammonium citrate and potassium citrate.
The citric acid serves to prevent tungstate from being deposited in the form of colloidal tungsten oxide. However, if the concentration of the citric acid is 0.5 g/l or less, as time goes by, tungsten oxide is deposited. If the concentration exceeds 10.0 g/l, plating is not affected. However, because the use of only 10.0 g/l of citric acid is sufficient for preventing deposition of colloidal tungsten oxide, the addition of citric acid of 10.0 g/l or more is not economical.
If citric acid is not added to the electrolyte of the present invention, or if 0.5 g/l or less of citric acid is added, tungsten in the electrolyte is present in the form of tungsten oxide. Therefore, tungsten in a zinc plating layer is present in the form of an oxide by physical reclamation or chemical adsorption during plating.
In the electrolyte of the present invention, almost all tungsten bonds with citric acid to form a complex compound. Specifically, almost all tungsten bonds with citric acid to form a complex compound in the electrolyte of the present invention, so as to prevent part or all of the tungsten from forming microcolloidal sludge. As a result, tungsten plating can be carried out with metallic tungsten.
The manner in which citric acid is added to the electrolyte for practicing the present invention is very important. If soluble tungstate is added to the electrolyte, followed by citric acid, part or all of the tungsten forms colloidal sludge. Subsequently, even though the citric acid is added, the produced tungsten sludge is not dissolved and thus is co-deposited in a plating layer, causing the deterioration of weldability of the plated steel sheet. Hence, the sequence of addition of the constituents is important.
With respect to the sequence of addition, in case citric acid precedes tungsten, or both of them are added simultaneously, part of the added tungsten inevitably forms sludge before reacting with the citric acid.
The present invention was brought to fruition by paying attention to this sequencing point. In accordance with the present invention, soluble tungstate and citric acid are dissolved in water, so that almost all of the tungsten can form a complex compound with the citric acid. That is, soluble tungstate and citric acid are simultaneously dissolved in water to sufficiently form a complex compound. Then, the addition of the complex compound to an electrolyte prevents tungsten from forming sludge before reacting with citric acid.
As plating proceeds, the tungsten concentration in an electrolyte reduces. When the reduced amounts of tungsten are replenished, citric acid and tungstate are dissolved in water to form a complex compound and then the solution containing the complex compound is injected into the electrolyte, thereby preventing formation of sludge.
In accordance with the present invention, in order to enhance the smoothness of a plating layer, polyethylene glycol is added. However, if the concentration of the polyethylene glycol is too little, the plating layer is roughened and thus its appearance is poor after plating. If the concentration is excessive, there is no problem in plating workability and the quality of a plated steel sheet. However, the addition of excessive concentration is not economical. Considering this fact, the concentration of the polyethylene glycol to be added is defined as 0.1-2.0 ml/l.
Preferably, a polyethylene glycol with molecular weight of 100-2000 is used.
Further, in accordance with the present invention, the concentration of an electric conductive aid in the electrolyte is defined as 30-400 g/l. The electric conductive aid serves to enhance electric conductivity of the electrolyte. As for continuous plating equipment operating in a high current density, at least 30 g/l of the electric conductive aid must be added, so as to manufacture products stably. If the concentration of the electric conductive aid exceeds 400 g/l, the electric conductive aid can be deposited when a temperature of the electrolyte is low.
In accordance with the present invention, potassium chloride, ammonium chloride, and sodium chloride, alone or a mixture, can be used as the electric conductive aid.
More preferably, the pH of the electrolyte is defined as 3-6. If the pH is below 3, plating efficiency is low, but if the pH exceeds 6, zinc ion and cobalt ion can be deposited in the form of hydroxide.
Preparation of Plated Steel Sheet
The plated steel sheet of the present invention can be readily manufactured by a general method comprising the steps of: using a conventional cold rolled steel sheet as a base iron, degreasing, washing, acid washing, and then electroplating the steel sheet in an electrolyte.
In other words, by electroplating a steel sheet using a conventional method in the electrolyte prepared as above, a plating layer consisting of Co: 0.1-3.0 wt %, W: 0.1-2.0 wt % and zinc: balance, is formed on the steel sheet. The tungsten plating is carried out with metallic tungsten. As a result, a Znxe2x80x94Coxe2x80x94W alloy electroplated steel sheet with excellent corrosion resistance and weldability can be manufactured in a stable manner.
The present invention is not limited to the illustrated plating conditions. Provided that a plating layer consisting of the aforementioned components can be obtained in the aforementioned electrolyte, any plating conditions can be within the range of the present invention.
Hereinafter, the present invention will be illustrated by way of examples.